A clean surface has a surface energy, which is the analog of the surface tension of a liquid. The surface energy of a solid, for example, affects how well the surface of the solid is wetted by adhesives and sealants.
The ability to monitor surface energy is useful in a variety of development and manufacturing applications, and several techniques to monitor surface energy have been established. Static and dynamic contact angle measurements between a standard solvent and the test surface are described in the measurement standard ASTM D5725-99 from the American National Standards Institute. It equates the surface energy of a test surface, or equivalently its surface free energy, to the surface tension of a liquid contacting the test surface with a zero degree contact angle. This technique is implemented in the OCA 15 video-based optical contact angle measurement system from Future Digital Scientific in Long Island, N.Y. The test surface is placed on the horizontal stage of the OCA 15, a liquid drop is placed on the surface, and images of the drop and surface are analyzed.
Peel test measurements are commonly done to measure adhesive joint quality. The ASTM D 1876 standard from the American National Standards Institute is a peel test for evaluating a bond between two flexible substrates. This and related techniques are implemented in the Motorized Peel Tester from Imada of Northbrook, Ill. Generally peel test results are weakly dependent on the surface energy of the adherends prior to bonding, since bond failure predominantly occurs in the adhesive and not at the interfaces between the adhesive and the adherends. However, for the case where the two adherends are of the same material, and when they are peeled apart they separate at their interface with no work being expended to distort the material, the work required to separate the two materials is twice the surface energy of the material.
Atomic force microscopy can measure surface energy directly by touching a probe tip to the surface under test and then measuring the force required to withdraw the tip from the test surface. This technique is implemented in the commercially available atomic force microscope from Hysitron of Minneapolis, Minn.
As the test surface becomes contaminated with atoms or molecules of dissimilar materials, the surface energy can change. For example, droplets of a vapor phase contaminant can wet and spread out on the surface if the surface tension of the liquid is less than the surface energy of the surface; the resulting contaminated surface will usually has a lower resulting surface energy.
Films or sub-monolayers of contaminating atoms or molecules on a surface are called molecular contamination. Such contamination has long been known to be a detractor in the application of paints, adhesives, and sealants. More recently, the importance of molecular contamination in display, semiconductor, and nano-materials fabrication has prompted the development of dedicated sensing techniques like the AiM-100 from Particle Measurement Systems of Boulder, Colo. This technique detects mass changes on the surface of a surface acoustic wave device; an increase in mass on the surface can be the signature of molecular contamination deposited on the surface.